U.S. patent application number 17/169200 was filed with the patent office on 2021-11-25 for frequency agile microwave radiometer, hyperspectral microwave radiometer and methods of operation.
The applicant listed for this patent is PHASE SENSITIVE INNOVATIONS, INC.. Invention is credited to Tom Dillon, Dennis Prather, Christopher Schuetz.
Application Number | 20210367678 17/169200 |
Document ID | / |
Family ID | 1000005767317 |
Filed Date | 2021-11-25 |
United States Patent
Application |
20210367678 |
Kind Code |
A1 |
Dillon; Tom ; et
al. |
November 25, 2021 |
FREQUENCY AGILE MICROWAVE RADIOMETER, HYPERSPECTRAL MICROWAVE
RADIOMETER AND METHODS OF OPERATION
Abstract
A hyperspectral radiometer may comprise one or more antennas, a
electro-optical modulator modulating the received RF signal onto an
optical carrier to generate a modulated signal having at least one
sideband; a filter filtering the modulated signal to pass the
sideband to a photodetector; and a photodetector producing an
electrical signal from which information of the RF signal can be
extracted. In some examples, the optical sideband may be spatially
dispersed to provide a plurality of spatially separate optical
components to the photodetector, where the spatially separate
optical components having different frequencies and correspond to
different frequencies of the received RF signal. In some examples,
the passed sideband may be mixed with an optical beam having a
frequency offset from the optical carrier to form a combined beam
having at least one optical signal component having a beat
frequency from which information of the RF signal can be
extracted.
Inventors: |
Dillon; Tom; (Newark,
DE) ; Schuetz; Christopher; (Avondale, PA) ;
Prather; Dennis; (New Castle, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PHASE SENSITIVE INNOVATIONS, INC. |
Newark |
DE |
US |
|
|
Family ID: |
1000005767317 |
Appl. No.: |
17/169200 |
Filed: |
February 5, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16365568 |
Mar 26, 2019 |
10917178 |
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17169200 |
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62648095 |
Mar 26, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 2210/006 20130101;
H04B 10/64 20130101; H04B 10/614 20130101; H04B 10/615 20130101;
H04B 10/675 20130101; H04B 10/5165 20130101 |
International
Class: |
H04B 10/61 20060101
H04B010/61; H04B 10/64 20060101 H04B010/64; H04B 10/516 20060101
H04B010/516; H04B 10/67 20060101 H04B010/67 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under
Contract No. 80NSSC18P2017 awarded by the National Aeronautics and
Space Administration (NASA). The government has certain rights in
the invention.
Claims
1-7. (canceled)
8. The method of claim 9, further comprising downconverting the
passed first sideband by impinging the combined beam on the
photodetector to extract an RF signal component having an RF
frequency of the beat frequency.
9. A method of RF signal processing comprising: receiving an RF
signal at an antenna; modulating the received RF signal onto an
optical carrier to generate a modulated signal having at least a
first sideband; filtering the modulated signal to pass the first
sideband to a photodetector; and extracting information of the RF
signal received by the antenna from an electrical signal generated
by the photodetector, wherein the method further comprises mixing
the passed first sideband with an optical beam having a frequency
that is offset from the optical carrier to form a combined beam
having at least one optical signal component having a beat
frequency, and altering the frequency of the optical beam.
10. (canceled)
11. A method of RF signal processing comprising: receiving a first
RF signal at an antenna; generating with a primary laser a primary
laser optical beam; splitting the primary laser optical beam to
provide a first portion of the primary laser optical beam as an
optical carrier and a second portion of the primary laser optical
beam; modulating the first RF signal onto the optical carrier to
generate a first modulated signal having a first sideband;
modulating the second portion of the primary laser optical beam
with a second RF signal to generate a second sideband; generating
with a secondary laser a secondary laser optical beam by locking
the secondary laser using a frequency component of the second
sideband; combining the first sideband with the secondary laser
optical beam to generate a combined optical signal; irradiating the
combined optical signal on a photodetector to generate an
electrical signal by the photodetector; filtering the electric
signal generated by the photodetector with a lowpass filter; and
extracting information of the first RF signal from the low pass
filtered electrical signal.
12. The method of claim 11, wherein the second sideband comprises a
plurality of harmonics, wherein generating the secondary laser
optical beam comprises selecting one of the plurality of harmonics
as the frequency component of the second sideband, wherein the
secondary laser locks onto the selected one of the plurality of
harmonics to generate the secondary laser optical beam having a
frequency of the selected one of the plurality of harmonics.
13. The method of claim 12, further comprising selecting a
different one of the plurality of harmonics for locking by the
secondary laser to generate the secondary laser optical beam with a
frequency of the selected different one of the plurality of
harmonics.
14. The method of claim 11, further comprising: generating the
second RF signal using a local oscillator; and adjusting the
frequency of the local oscillator to alter the frequency of the
frequency component of the second sideband to thereby also alter
the frequency of the secondary laser optical beam.
15. The method of claim 11, wherein the secondary laser optical
beam is generated with a frequency that is offset from the
frequency of the primary laser optical beam by a first offset
value, and wherein the first offset value falls within the RF
frequency band of the operational frequencies of the antenna.
16. The method of claim 15, wherein the first offset value is
adjustable.
17. The method of claim 15, further comprising adjusting the first
offset value by sweeping the frequency of the second RF signal.
18. The method of claim 17, further comprising generating the
second RF signal with a local oscillator, wherein sweeping the
frequency of the second RF signal comprises adjusting the local
oscillator.
19. The method of claim 16, further comprising: measuring the power
of the low pass filtered electrical signal, wherein the measured
power represents the power of a first frequency band of the first
RF signal received at the antenna; and modifying the first offset
value to in order that the measured power represents the power of a
second frequency band of the first RF signal received at the
antenna.
20. The method of claim 11, wherein the lowpass filtered electrical
signal corresponds to a selected frequency band of the first RF
signal received at the antenna.
21. The method of claim 20, wherein the magnitude of a D.C. voltage
output by the photodetector represents power of the selected
frequency band of the first RF signal received at the antenna.
22. The method of claim 20, further comprising adjusting the first
offset value to alter the selection of the frequency band of the
first RF signal.
23. The method of claim 16, further comprising: generating the
second RF signal using a local oscillator; and adjusting the first
offset value by adjusting the local oscillator.
24. The method of claim 23, wherein adjusting the local oscillator
modifies the frequency of the second RF signal to cause the
frequency of the secondary laser optical beam to be modified in
response thereto.
25. The method of claim 19, wherein the lowpass filtered electrical
signal corresponds to a selected frequency band of the first RF
signal received at the antenna, the selected frequency band having
a bandwidth of 10 to 1000 MHz.
26. The method of claim 11, wherein the secondary laser optical
beam is generated with a frequency that is offset from the
frequency of the primary laser optical beam by a first offset
value, and wherein combining the first sideband with the secondary
laser optical beam generates a combined optical signal having one
or more beat frequencies respectively corresponding to one or more
component frequencies of the first RF signal.
27. The method of claim 26, wherein the photodetector generates the
electrical signal having frequency components corresponding to the
one or more beat frequencies.
28. The method of claim 27, further comprising adjusting the first
offset value to shift the one or more beat frequencies by an amount
corresponding to the adjustment.
29. The method of claim 9, wherein the optical beam is phase locked
to the optical carrier.
30. The method of claim 21, wherein the steps of claim 21 are
performed to operate a radiometer having spectral coverage in both
millimeter and microwave frequencies.
31. The method of claim 30, wherein the radiometer has a spectral
coverage of at least 20 GHz to 300 GHz.
32. The method of claim 30, wherein the radiometer is formed as a
spectrograph.
33. The method of claim 30, wherein the first RF signal comprises
an atmospheric transmission of plural RF frequency components, and
wherein receiving the first RF signal comprises passive sensing of
the first RF signal to obtain atmospheric condition data.
34. The method of claim 33, wherein the atmospheric condition data
comprises at least one of atmospheric temperature data and
atmospheric moisture data.
35. The method of claim 30, wherein the first RF signal comprises
an atmospheric transmission of plural RF frequency components, and
wherein receiving the first RF signal comprises passive sensing of
the first RF signal to obtain data of at least one of precipitation
rate, land surface emissivity, snow cover, sea ice concentration,
land surface temperature and cloud liquid water.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Divisional of U.S. application Ser.
No. 16/365,568 filed Mar. 26, 2019, which is a Nonprovisional
Application of Provisional Patent Application No. 62/648,095 filed
Mar. 26, 2018, the contents of each which are hereby incorporated
by reference in its entirety.
BACKGROUND
[0003] Passive microwave remote sensing is currently utilized by
NASA, NOAA, ESA and others to conduct Earth Science missions,
including weather forecasting, early warning systems, and climate
studies. Humidity and temperature sounding is conducted near
several absorption lines to determine key parameters of the
atmospheric state, including moisture content, temperature profile,
and barometric pressure. Using neural networks, these parameters
are retrieved from raw sensor data at a small number of discreet
frequencies. To improve retrieval accuracy as well as predictive
ability of weather models, measurements at a large number of
closely spaced frequencies, i.e. hyperspectral sensing, should be
implemented. A hyperspectral radiometer would also be helpful for
the study of atmospheric composition and dynamics on other
celestial bodies such as Jupiter or Titan, as well as other
applications, such as for use in RF interference mitigation and
sensor calibration.
[0004] Microwave/millimeter-wave radiometry has demonstrated
tremendous utility in space-based meteorological data-gathering for
many decades. Current efforts in microwave sensing encompass a wide
frequency range of operation--from sounders operating in the low
MHz range to millimeter-wave (mmW) radiometers and radars operating
in the hundreds of GHz range. The atmospheric transmission of
infrared and microwave RF frequencies in the GHz range (e.g., from
300 MHz-500 GHz and beyond) are often of interest. Within this
spectrum, there exist several distinct absorption lines where
microwave sounders are typically deployed. The transmission, or the
related sky brightness temperature, near these absorption lines is
a strong function of atmospheric conditions, such as moisture.
[0005] Much can be discerned about the atmosphere, ground cover,
and ocean surface by monitoring these signals. The raw sensor data
is collected at a number of discrete frequencies from which many
useful data products are retrieved, commonly using neural networks.
Besides the atmospheric temperature and moisture profile,
additional data products include precipitation rate, land surface
emissivity, snow cover, sea ice concentration, land surface
temperature, cloud liquid water, and more. These data products are
provided to the National Weather Service for weather forecasting,
as well as to the larger scientific community.
[0006] However, radiometers systems are often bulky and limited to
detecting a narrow range of frequencies. Resolution and power
requirements may also make current systems impractical and/or less
than optimal for many implementations.
SUMMARY
[0007] Exemplary embodiments provide a radiometer and hyperspectral
sensing methods to process RF signals received by one or more
antenna elements.
[0008] According to aspects of various embodiments, a method of RF
signal processing comprises receiving an incoming RF signal at an
antenna; modulating the received RF signal onto an optical carrier
to generate a modulated signal having at least one sideband;
filtering the modulated signal to pass the sideband to a
photodetector; and extracting information of the RF signal received
by the antenna from an electrical signal generated by the
photodetector. The method may comprise spatially dispersing the
passed sideband to provide a plurality of spatially separate
optical components to the photodetector, the spatially separate
optical components having different frequencies. The method may
comprise mixing the passed sideband with an optical beam having a
frequency offset from the optical carrier to form a combined beam
having at least one optical signal component having a beat
frequency.
[0009] According to aspects of the various embodiments, a
hyperspectral radiometer may be implemented and configured to
perform one or more of such operations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present disclosure now will be described more fully with
reference to the accompanying drawings, in which various
embodiments are shown.
[0011] FIG. 1 is a block diagram of an exemplary radiometer in
accordance with aspects of the invention;
[0012] FIG. 2 is a block diagram of an exemplary hyperspectral
radiometer in accordance with aspects of the invention;
[0013] FIG. 3 is a block diagram of an exemplary implementation of
the hyperspectral radiometer of FIG. 2;
[0014] FIG. 4 shows details of an exemplary implementation of the
AWG dispersion element 60' of FIG. 3;
[0015] FIG. 5 illustrates another example of a hyperspectral
radiometer according to embodiments;
[0016] FIG. 6 shows a modification of the hyperspectral radiometer
of FIG. 5 that may also be applied to the other disclosed
embodiments; and
[0017] FIG. 7 is an illustration to help explain generation of a
beat frequency.
[0018] FIG. 8 provides exemplary details of post processing the
downconverted signal provided by photodetector.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0019] The present disclosure now will be described more fully
hereinafter with reference to the accompanying drawings, in which
various exemplary embodiments are shown. The invention may,
however, be embodied in many different forms and should not be
construed as limited to the exemplary embodiments set forth herein.
These example exemplary embodiments are just that--examples--and
many embodiments and variations are possible that do not require
the details provided herein. It should also be emphasized that the
disclosure provides details of alternative examples, but such
listing of alternatives is not exhaustive. Furthermore, any
consistency of detail between various exemplary embodiments should
not be interpreted as requiring such detail--it is impracticable to
list every possible variation for every feature described herein.
The language of the claims should be referenced in determining the
requirements of the invention.
[0020] Ordinal numbers such as "first," "second," "third," etc. may
be used simply as labels of certain elements, steps, etc., to
distinguish such elements, steps, etc. from one another. Terms that
are not described using "first," "second," etc., in the
specification, may still be referred to as "first" or "second" in a
claim. In addition, a term that is referenced with a particular
ordinal number (e.g., "first" in a particular claim) may be
described elsewhere with a different ordinal number (e.g., "second"
in the specification or another claim). As used herein, the term
"and/or" includes any and all combinations of one or more of the
associated listed items.
[0021] It will be understood that when an element is referred to as
being "connected" or "coupled" to or "on" another element, it can
be directly connected or coupled to or on the other element or
intervening elements may be present. In contrast, when an element
is referred to as being "directly connected" or "directly coupled"
to another element, or as "contacting" or "in contact with" another
element, there are no intervening elements present.
[0022] The terminology used herein is for the purpose of describing
particular embodiments and is not intended to be limiting of the
inventive concept disclosure and claims. As used herein, the
singular forms "a," "an" and "the" are intended to include the
plural forms as well, unless the context clearly indicates
otherwise. It will be further understood that the terms
"comprises," "comprising," "includes" and/or "including," when used
herein, specify the presence of stated features, integers, steps,
operations, elements, and/or components, but do not preclude the
presence or addition of one or more other.
[0023] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art of this disclosure.
It will be further understood that terms, such as those defined in
commonly used dictionaries, should be interpreted as having a
meaning that is consistent with their meaning in the context of the
relevant art and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
[0024] Hereinafter, example embodiments will be explained in detail
with reference to the accompanying drawings. The same reference
numerals will be used to refer to the same elements throughout the
drawings and detailed description about the same elements may be
omitted in order to avoid redundancy.
[0025] Aspects of the embodiments provide a signal detection
mechanism wherein RF signals received by one or more antennas are
upconverted by fiber-coupled optical phase modulators driven by the
antenna element(s). The conversion results in sidebands on an
optical carrier wave supplied by a laser. These optical sidebands
are substantially proportional in power to the RF power incident
into the antenna element(s), and also preserve the phase carried by
the incident RF signals. The optical sidebands can be used to
analyze the RF energy received by the antenna(s).
[0026] FIG. 1 illustrates an exemplary embodiment of a radiometer.
An incoming RF signal is provided by antenna 20 via an RF waveguide
4 to electro-optical modulator 30 (also referenced herein as an EO
modulator). The incoming RF signal may be of any frequency band of
interest and the antenna may be optimized to capture an
electromagnetic wave of such frequencies (e.g., radiating arm(s) of
the antenna may have a length of one half of the wavelength of
within the RF frequency band of the operational frequency of the
antenna 20). The RF signal may be transmitted from the antenna 20
by RF waveguide 4 to electro-optical modulator 30. Optionally, an
amplifier (not shown in FIG. 1) may be provided between the antenna
20 and the electro-optical modulator 30 to amplify the RF signal
output by the antenna 20 and transmit the same to the phase
modulator 30. In some examples, the EO modulator 30 and waveguide 4
may be provided on a semiconductor chip.
[0027] When plural EO modulators 30 are used (as described
elsewhere herein), plural EO modulators 30 and plural RF waveguides
4 may be provide with the same chip. The on chip RF waveguide(s) 4
may be coplanar waveguide(s) (CPW). The electro-optical modulator
30 receives the RF signal and modulates an optical carrier provided
by laser 10 (e.g., a laser beam of frequency .omega..sub.o) on
optical fiber 2a to generate sidebands (at
(.omega..sub.o-.omega..sub.m) and (.omega..sub.o+.omega..sub.m))
that preserve the signal amplitude and phase of the RF signal. The
electro-optical modulator 30 may be considered a mixer and be
implemented with a phase modulator that modulates the phase of the
optical carrier provided by laser 10 in response to the received RF
signal. The received RF signal is thus upconverted into the optical
domain. The RF signal received in the RF domain appears in the
optical domain (in the optical signal output by the EO modulator
30) as sidebands of the optical carrier frequency of optical
carrier of the laser 10. This up-conversion of the RF signal into
optical domain is coherent in the sense that all the phase and
amplitude information present in RF is preserved in the optical
sidebands. This property of coherence preservation in optical
up-conversion allows the recovery of the RF signals and/or
information thereof (such as power) using optical means as
described herein.
[0028] In some examples, the electro-optical modulator 30 may be a
lithium niobate modulator. An optical filter 40 receives the
modulated optical carrier on optical fiber 2b and filters the same
to pass one of the sidebands on to a photodetector (blocking the
optical carrier and other sideband). The filter 40 may be a
conventional DWDM (dense wavelength-divisional multiplexing)
filter. The passed sideband is received by a photodetector 50
(e.g., a photodiode, such as a PIN photodiode) on optical fiber 2c.
Photodetector 50 generates a photocurrent proportional to the
incident optical power received from the passed sideband. Thus, the
radiometer acts as a microwave power meter, producing an output
signal proportional to the input power.
[0029] The filter 40 may be a passband filter to narrow the
bandwidth a particular detection frequency and thus detect the
microwave power of a desired frequency. In some examples, the
passband filter is tunable to achieve a frequency-agile
receiver/radiometer in which received power of a selected frequency
or a selected frequency band (within the larger operational
frequency bandwidth of the antennal and/or radiometer) may be
measured. The detection bandwidth may be limited by the ability to
couple the RF signal to the EO modulator; in practice, the antenna
or amplifier inserted between the modulator and antenna (not shown
in FIG. 1) would restrict the bandwidth. However, use of such an
amplifier is optional and may be used depending on design
implementations. In addition, multiple frequency bands may be
measured with a single back-end by channelizing the receiver and/or
cascading the modulators. Thus, the use of the ultra-wideband EO
modulators may realize spectral coverage in a wide range, e.g., 20
to 300 GHz or more, in both millimeter and microwave frequencies,
etc. The use of fiber optics offers low loss, dispersion free
routing of signals with optical fiber waveguides (e.g., 2a, 2b and
2c) that are much less heavy and bulky as compared to metal
waveguides. Furthermore, the upconversion of the RF signal by the
EO modulator 30 preserves both amplitude and phase of signal, which
enables coherent detection. In some examples, an imager employing
aperture syntheses is formed from a coherent array of these
receiver elements.
[0030] FIG. 2 illustrates an example of a hyperspectral radiometer
according some embodiments. In the example of FIG. 2, the
hyperspectral radiometer is formed as a microwave spectrograph.
[0031] Use of same or similar reference numerals refer to same
structure described elsewhere herein and repetitive description may
be omitted. In the example illustrated in FIG. 2, the hyperspectral
radiometer includes laser 10, antenna 20, EO modulator 30, filter
40 and optical fibers 2a, 2b and 2c, connected and functioning as
described with respect to FIG. 1. The hyperspectral radiometer of
FIG. 2 shows use of amplifiers 22 to amplify the RF signal provided
by antenna 20, with RF waveguides 4 providing RF signal connections
between the antenna 20, the amplifiers 22 and the EO modulator 30.
The antenna 20, the amplifiers 22 and the EO modulator 30 may be
formed on a printed circuit board (RF PCB substrate 24), such as
being fabricated by patterning a liquid crystal polymer material
layer of the RF PCB substrate using standard PCB manufacturing
techniques. Other active components, such as low noise amplifiers
and other passive components, such as RF signal splitters and
combiners, may also be formed from the RF PCB substrate 24.
[0032] As discussed with respect to FIG. 1, filter 40 may pass one
of the optical sidebands generated by the EO modulator 30 and
transmit the same on optical fiber 2c. As shown in FIG. 2, the
sideband transmitted by optical fiber 2c is input into an optical
dispersion element 60. Optical dispersion element 60 divides the
light of the received sideband according to wavelength of the
component elements forming the sideband. For example, the optical
dispersion element 60 may a prism, a diffraction grating, a spatial
light modulator, an arrayed waveguide grating (AWG), a plurality of
these elements and/or a combination of some or all thereof. The
output of the optical dispersion element 60 is the received optical
sideband having its optical frequency (and corresponding optical
wavelength) components spatially separated. The spatially separated
frequency components may be transmitted to detector 50' through any
suitable optically transmissive medium, such as free space, lenses,
etc. Detector 50' may be an array of photodetectors and may be
embodied as a 1D array of photodetectors or as a 2D array, such as
a conventional image sensor of a camera formed from a plurality of
rows and columns of pixels (each pixel comprising a photodetector,
such as a photodiode configured to sense the intensity of light
impinged thereon). As detector 50' receives the frequency
components of the optical sideband, the detector 50 may determine
the corresponding intensities of each of the frequency components
of the optical sideband, such as by providing pixel intensity data.
For example, if detector 50' comprises 1000 columns of pixels, the
average pixel intensity of each column may correlate to and
represent the intensity of a corresponding frequency of the optical
sideband, which in turn, may represent the corresponding energy of
a particular frequency of the RF signal received by the antenna 20.
Of course, rows of pixels may be used rather than columns or other
groupings of pixels dependent upon the dispersion of the optical
sideband provided by the dispersion element 60.
[0033] Thus, the hyperspectral radiometer may simultaneously detect
the intensities of the different frequencies of the RF signal
received by the antenna 20 in real time. It will be appreciated
that reference to different frequencies here in actuality
encompasses a range of frequencies within a relatively very narrow
band. Using an optical carrier of 1550 nm from laser 10, a 1 GHz
resolution requires a resolving power of .about.200000, which is
challenging. A diffraction grating may require a large area to
achieve that kind of resolving power. However, an array waveguide
grating (AWG) provides similar results to a diffraction grating in
operation but uses guided optics rather than free space.
Commercial, off the shelf (COTS) AWG can currently provide
resolution of 25 GHz, while more advanced AWG can achieve .about.3
GHz resolution. Thus, the hyperspectral radiometer may
simultaneously determine power of the frequency components of the
received RF signal with a resolution of 25 GHz or less per detected
frequency, such as with a resolution of less than 3 GHz, such as 1
to 3 GHz or even finer (e.g., below 0.1 GHz for some applications),
depending on the limits of the AWG.
[0034] FIG. 3 illustrates details of one example implementation of
the hyperspectral radiometer of FIG. 2 using optical fibers 2d to
form the AWG grating. Dispersion element 60' is formed as an array
waveguide grating (AWG) including optical splitter 60a and a
plurality of optical fibers 2d of different lengths. Optical
splitter 60a receives the optical sideband from filter 40 via
optical fiber 2c and splits the optical sideband m-ways into a
plurality of channels, which are then fed to corresponding ones of
m optical fibers 2d. As shown in in FIG. 3, each of the optical
fibers 2d may have different length from each of the other optical
fibers 2d. For example, the optical fibers may substantially
correspond to the optical path length from the input of the optical
splitter 60a to the input into camera 50'. For m channels, the
optical path length of the channels may increment channel by
channel by the same amount. For example, for m channels, the
optical path length of the ith channel (where i is a number between
1 and m) may be x+.DELTA.i where x is a constant and .DELTA. is a
fixed increment. The outputs of the optical fibers 2d may be
arranged regularly spaced apart (e.g., linearly spaced apart at a
fixed pitch). As shown in FIG. 3, the optical fibers 2d may be
arranged (at their outputs) in a physical order corresponding to a
numerical order of their lengths (and corresponding to the optical
path lengths of the corresponding optical channels that they form).
In other examples, the outputs of the optical fibers 2d may be
spaced apart at irregular spacings.
[0035] FIG. 4 shows details of an exemplary implementation of the
AWG dispersion element 60' of FIG. 3 (repetitive descriptions of
the elements shown therein may be omitted). As will be appreciated,
by providing different optical path lengths between the end of
optical fiber 2c (corresponding to the input of the splitter 60a),
the optical fibers 2d provide the m portions of the optical
sideband with different time delays (e.g., at a fixed increment
between the 1.sup.st to mth optical channels formed in part by the
m optical fibers 2d). Thus, phase differences may thus be provided
by the m optical channels for the frequency components of the
optical sideband. It will be appreciated that phase differences
provided at the output of the m optical fibers 2d (corresponding to
the end of the m optical channels) are dependent on
frequency/wavelengths of the frequency components of the optical
sidebands. As such, each frequency component is provided with a
different phase offset between each neighboring optical fiber and
the coherent interference of each frequency component (in the
transmissive/transparent medium 60b (free space, glass, etc.) at
the output of the optical fibers 2d) thus acts to direct each
frequency component in a different direction from other frequency
components of the optical sideband, as shown in FIG. 4. Thus, the
AWG acts to spatially separate the frequency components of the
optical sideband. The spatially separated frequency components are
then directed to detector 50'. FIG. 4 shows an example where each
optical fiber of a set of optical fibers 2e separately transmits a
corresponding frequency component to detector 50', however such
additional structure may be avoided and the detector 50' (e.g., the
surface of a conventional CMOS image sensor) may be provided at the
output of medium 60b.
[0036] In order to eliminate environmental factors that induce
phase variations, such as acoustic noise and thermal drift, the
fiber channels should be phase locked formed by each optical fiber
2d. Phase modulators (not shown) may be inserted within each
optical channel (e.g., at the beginning or end of each of the
optical fibers 2d) to stabilize each channel.
[0037] As the sensed intensity of the frequency components of the
optical sideband correspond to a corresponding power level of an RF
frequency, they hyperspectral radiometer may simultaneously detect
plural RF frequencies in real time. For example, using only 8
channels, the entire frequency range from 50-75 GHz may be detected
with .about.3 GHz spacing or less in resolution. Spectral
resolution can be tailored by adjusting the fiber lengths in the
AWG 60' to adjust the optical path lengths. If finer resolution is
desired, operation could be limited to one side of the absorption
edge (e.g., of the antenna), e.g., from 60-75 GHz, and/or
additional channels may be provided.
[0038] FIG. 5 illustrates another example of a hyperspectral
radiometer according to various embodiments. Use of same or similar
reference numerals refer to same structure described elsewhere
herein and repetitive description may be omitted. In the example
illustrated in FIG. 5, the hyperspectral radiometer includes laser
10, antenna 20, EO modulator 30, filter 40, optical fibers 2a, 2b
and 2c, and RF waveguide 4 connected and functioning as described
with respect to FIGS. 1 and 2. As shown in FIG. 5, laser 10 is
provided as a primary laser and generates the optical carrier which
is then provided to the EO modulator 30 via optical fiber 2a for
electro-optical modulation by the EO modulator 30 and RF signal
(provided by RF waveguide 4). The modulated optical carrier, with
optical sidebands corresponding to the RF signal, is thus provided
to filter 40 which passes one of the sidebands to impinge on
detector 50, as described herein.
[0039] The embodiment of FIG. 5 also provides a second laser 11
that provides an optical beam that operates at a frequency offset
from the frequency of the first optical beam (the optical carrier
described herein) generated by the primary laser 10. In the example
of FIG. 5, primary laser 10 generates an optical carrier having a
frequency of W.sub.0 and secondary laser 11 generates a second
optical beam having a frequency of W.sub.0+an offset (N.OMEGA.).
The second optical beam is combined with the optical sideband
output by the filter 40 and also impinges on detector 50 (as a
combined optical beam, combined with the optical sideband).
[0040] Once the RF signal from antenna 20 has been upconverted to
optical sidebands using the EO modulator 30 (fed by the RF signal
from antenna 20 and the optical carrier from laser 10), one of
those sidebands (output by filter 40) is combined with the optical
beam from the second laser 11 that is offset in frequency from the
first laser by the desired receiver detection frequency. The
desired receiver detection frequency may be selected and modified.
When two optical signals of different frequencies are coherently
combined, the optical signals constructively and destructively
interfere with one another to create a combined optical signal
having a beat frequency corresponding to the difference in
frequencies of the combined optical signals. FIG. 7 is provided to
explain the concept of a beat frequency, showing the upper two
signals having a wavelength (frequency) of .lamda..sub.1(f.sub.1)
and .lamda..sub.2 (f.sub.2), respectively, that, when combined
(lower signal) provide a signal provides a beat frequency of
f.sub.beat (equal to the difference of f.sub.1 and f.sub.2). The
beat frequency is the frequency of the oscillation of the envelope
of the combined signal.
[0041] When a combined optical signal having such a beat frequency
is irradiated on the photodetector 50, the photodetector 50
generates an electrical signal at the beat frequency. In this
example, each component frequency of the optical sideband provided
by filter 40 on optical waveguide 2c is combined with the secondary
optical beam from laser 11 resulting in a beat frequency for that
component frequency (which are all combined together at the
photodetector) that may be extracted as a corresponding frequency
component of the electrical signal by the photodetector 50. Thus,
together with the photodetector 50, the secondary optical beam
provided by laser 11 acts to downconvert each of the frequency
components of the optical sideband provided by filter 40 by a
frequency equal to the frequency of the secondary optical beam. The
resulting multi-frequency electrical signal generated by
photodetector 50 thus contains frequency components representing
the frequency components of RF signal obtained by antenna 20 (as
provided by the optical side band output by filter 40).
[0042] The difference in the frequencies of the primary optical
beam (from laser 10) and the secondary optical beam from laser 11
may be set by tuning the second laser 11. Specifically, the
frequency of the second laser 11 may be selected to correspond to
the desired detection frequency. The first laser 12 may provide an
optical beam that is fixed in frequency to align the desired
sideband to pass through the filter 40 while suppressing the
optical carrier frequency (corresponding to the frequency of laser
10, which might otherwise saturate the receiver back-end. When the
second laser 11 is aligned in frequency with the optical sideband,
the sideband signal is converted to an intermediate frequency
signal via the photodetector 50.
[0043] The optical heterodyne (mixing of two optical beams of close
frequency) downconversion in this case produces beat signals for
all frequencies passed through the filter 40. For example, if the
offset between the two optical beams of the lasers 10 and 11 is 100
GHz, and the RF signal received by the antenna 20 is between 90 and
120 GHz, then the electrical signal generated by the photodiode 50
may contain signal components at all the different RF frequencies
of the signal received by the antenna 20. In this example, the RF
signal received by the antenna 20 in the RF frequency spectrum of
90 to 120 GHz (represented by the optical sideband from filter 40)
may be downconverted by the optical carrier frequency plus an
additional 100 GHz to provide corresponding RF electrical signal
components within the frequency range of -10 to 20 GHz (where the
negative frequencies is folded back around DC to be confounded with
the signal from 100 to 110 GHz). By placing an RF filter to the RF
signal output by the photodetector 50, further resolution is
possible.
[0044] For example, referring to FIG. 8, a low pass filter (not
shown in FIG. 5) of 100 MHz may be provided after the photodetector
50 (e.g., just before RF detector 80) and may thus select a very
narrow frequency range (e.g., of 200 MHz) of the received RF
frequency (i.e., centered around 100 GHz in this example) whose
power may then be detected with the RF detector 80. The magnitude
of a D.C. voltage output by this RF detector 80 may correspond to
this selected narrow frequency range of the received RF signal and
be captured and digitized by analog to digital converter 90. The
optical processing here thus allows use of simple electronics
designed to operate at DC-100 MHz to easily provide high spectral
resolution using electronic components that are cheap and readily
available. By modifying the frequency of the optical beam of the
second laser 11, a different narrow frequency range of the of the
received RF signal may be selected and analyzed (e.g., such as
determining its power). The frequency of the optical beam of the
second laser 11 may thus be repetitively modified in this fashion
to analyze very narrow frequency ranges of the received RF signal,
providing a broadband operation of the hyperspectral radiometer
over hundreds of gigahertz, only limited by the tuning of the
second laser.
[0045] The operation thus described has moved a millimeter wave RF
signal at (30-300) gigahertz frequencies to optical (200 terahertz)
frequencies using an electro-optic modulator, and then to
microwave/radio wave frequencies (10-1000 megahertz), using optical
heterodyne downconversion, the where it can conveniently be
filtered and finally square law detected to extract the desired
baseband signal. The resulting signal is proportional to the power
in the millimeter wave spectrum at a particular frequency and
spectral resolution, both of which are easily adjusted. By sweeping
the frequency of the optical LO 13 and/or selecting different
harmonics to lock the secondary laser 11, a set of measurements
across large swaths of the electromagnetic spectrum can be
generated, thus realizing a frequency agile, tunable, hyperspectral
receiver.
[0046] The secondary laser 11 may run free of any synchronization
with primary laser 10. However, the second laser 11 may also be
locked to provide an optical beam with a frequency having a
constant offset from the frequency of the optical beam of the
primary laser 10 (an offset that is adjustable). In FIG. 5, this
adjustable constant frequency offset is accomplished using a
tunable optical paired source, or TOPS 70. As shown in FIG. 5, the
optical beam from primary laser 10 is split by beam splitter with a
portion provided to EO modulator 14 where it is modulated by an RF
signal generated by local oscillator 13 to generate several
harmonic frequencies (offset from the optical carrier frequency of
the primary laser optical beam by an integer multiple of the RF
frequency of the local oscillator 13). The optical carrier
component of the modulated signal is then removed by filter 15 and
provided to optical circulator 16. Secondary laser 11 is also fed
to optical circulator 16 where it is thermally locks on one of the
harmonics resulting from the local oscillator to thus provide an
optical beam of a known and selectable frequency offset from the
optical carrier frequency of the primary laser 10. Selection of the
frequency of the laser 11 may be performed during operational
startup of the secondary laser by biasing temperature and/or by
adjusting the RF frequency of the local oscillator 13. Such
selection and tunability of the frequency of secondary laser 11 is
described in U.S. Pat. No. 9,525,489, the contents of which are
hereby incorporated by reference.
[0047] FIG. 6 shows a modification of the hyperspectral radiometer
of FIG. 5 that may also be applied to the other embodiments
described herein (e.g., with respect to the embodiments of FIGS. 1,
2 and 3). As shown in FIG. 6, plural antennas 20a, 20b and 20c
having different operational frequencies are used to obtain several
RF signals that are respectively modulated by separate EO
modulators 30a, 30b and 30c. In this example, the EO modulators
30a, 30b and 30c are cascaded together. "Downstream" EO modulators
20b and 20c modulate the outputs of the "upstream" EO modulators,
20a and 20b respectively. As these outputs contain an optical
carrier component, the additional modulations of the downstream
modulations are additive to the overall sideband signal to provide
a sideband signal having power information of the RF frequency
components of the RF signals captured by each of the different
antennas 20a, 20b and 20c. Alternative to the cascading
configuration of FIG. 6, plural EO modulators 30a, 30b and 30c may
each receive a split component of the optical carrier (e.g.,
directly from an optical splitter receiving the optical carrier
output by the primary laser 10). In this instance, the outputs by
the plural EO modulators 30a, 30b and 30c may be combined by an
optical combiner and provided to filter 40 via optical fiber
2b.
[0048] The foregoing is illustrative of exemplary embodiments and
is not to be construed as limiting thereof. Although a few
exemplary embodiments have been described, those skilled in the art
will readily appreciate that many modifications are possible
without materially departing from the novel teachings and
advantages of the inventive concepts. For example, although various
components and optical connections therebetween have been shown
separately, it will be appreciated that such (some or all)
components may be combined on a single chip as part of a PIC
(photonic IC). For example, although the RF signal received by the
antenna and processed by the radiometer has mostly been referenced
as a microwave/millimeter wave RF signal, it will be appreciated
that other portions of the electromagnetic spectrum (e.g., RF
signals other than microwave/millimeter wave) and may be processed
by the radiometer. It may be appreciated that the same backend
optical processing of the radiometers described herein may be used
with different spectrum by swapping out the antennas 20 to use
antennas with different operational frequencies (different RF
waveguides 4 may also need to be implemented in certain cases).
Accordingly, all such modifications are intended to be included
within the scope of the present invention as defined in the
claims.
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